(1) Field of the Invention
The present invention relates to an improvement in a semiconductor memory device called an "EPROM" (erasable programmable read only memory).
(2) Description of the Prior Art
In most conventional EPROM devices, information is electrically written or stored, and the written or stored information can be erased by irradiation with ultraviolet rays so that repeated use of the device is possible.
Recently, research has been performed on EPROM devices in which both the writing and erasing of information can be performed electrically. As these EPROM devices can be handled very easily, the structure, especially the structure of the package, can be simplified. For example, U.S. Pat. No. 3,825,946 discloses the EPROM device which can be charged or discharged electrically. In this device, a second gate and a third gate are adapted so as to cooperatively enable a discharge to take place from a floating gate due to avalanche injection through an insulating layer having a thickness of about 500 to 1,000 angstroms. To discharge the floating gate, a positive pulse of a high voltage (e.g., approximately 35 volts) is applied to the second gate, with the third gate and the substrate grounded. To charge the floating gate, a positive pulse of a similarly high voltage is applied to the second and third gates, with the substrate grounded. However, application of a high voltage at a time during which the floating gate is charged and discharged is liable to cause a breakdown. If the floating gate is charged by applying a low voltage, no breakdown is caused even when a high voltage is applied to discharge the floating gate. However, in this case, two electrodes for high and low voltages, respectively, must be provided. If both a charge and a discharge of the floating gate is conducted at a relatively low voltage, reliability of the writing and erasing would be reduced.